Such machines are often referred to by the initials FFS for "form, fill, and seal" machine.
Numerous machines of this type have already been proposed (see for example documents EP-A-528 721 and U.S. Pat. No. 4,894,975.
Most such machines comprise a forming neck which receives at its input the film in the flat state taken from a payout stand and which delivers at its output the film shaped into a tube, a filler chute opening out into the forming neck and consequently into said tube, means for feeding closure means onto the film and for fixing them to the film, longitudinal heat-sealing means for closing the tube longitudinally, and means suitable for generating sequentially a first transverse line of heat-sealing before a product is inserted into the tube via the filler chute, and then a second transverse line of heat-sealing after the product has been inserted into the tube, in order to close a package around the product.
Still more precisely, most machines that have been proposed so far are designed to receive their closure strips in the longitudinal direction, i.e. parallel to the film travel direction. However, such machines with longitudinal strips suffer from the drawback of limiting the height of the resulting bags. This height is equal to half the circumference of the forming neck. In addition, packages obtained on such machines sometimes leak. This is a result in particular of the fact that the operation of the transverse heat-sealing means is disturbed by the extra thickness formed by the longitudinal strips.
Numerous attempts have been made to improve the above situation, by placing the closure strips not longitudinally, i.e. parallel to the travel direction of the film, but transversely, i.e. perpendicular to the travel direction of the film.
The earliest attempt on those lines known to the inventors is described in documents U.S. Pat. No. 4,617,683 and U.S. Pat. No. 4,655,862. Those documents, which are more than 10 years old, propose two solutions for bringing the transverse strips onto the film while it is in the flat state prior to reaching the forming neck. The first solution consists in moving segments of closure strip laterally by drive wheels placed beside the edge of the travelling film, acting in combination with a transversely movable suction head. The second solution consists in placing closure strips on the film by using a rotary cylinder fitted with pneumatically controlled temporary holding means.
Those proposals were found initially to be quite promising. Unfortunately, they did not give satisfaction and they have been abandoned by the person skilled in the art. In particular, it was found that the means described in the above-mentioned documents did not always ensure that, when placed on the film, the strips were in an accurately rectilinear state. Consequently, it was often difficult to make the complementary strips cooperate with each other.
Other solutions were then proposed in an attempt to overcome the drawbacks that resulted from implementing the means described in the above-mentioned documents.
For example, in document U.S. Pat. No. 4,666,536, it is proposed to wind a part of the film on a core and to bring a closure strip segment extending across the width of the film onto said wound part of the film by means of a heating tunnel that serves to fix the strips. Theoretically, the means described in that document have the advantage of enabling forming to take place with a continuously travelling film. Unfortunately, such means have turned out to be very complex and difficult to implement.
Document U.S. Pat. No. 4,701,361 proposes forming a film which is provided sequentially with complementary closure strips by extruding a tubular film fitted with helically wound closure strips that are extruded therewith or that are applied thereto, and then cutting the tubular film helically to obtain a tape that is provided with uniformly distributed transverse strips.
To reduce the length of the strips deposited on the film, and consequently to improve the positioning thereof, document U.S. Pat. No. 4,878,987 proposes supplying the strips not from a supply on one side only, as described in document U.S. Pat. No. 4,617,863 and U.S. Pat. No. 4,655,862, but from two supplies of strips disposed on respective sides of the film that is to form the bags. Thus, each of the two strip segments displaced over the film, from the respective supplies on either side, no longer covers the entire width of the film, but only half the width.
Thereafter, document U.S. Pat. No. 4,844,759 proposed two other solutions. The first proposed solution consists in driving the closure strips by means of an endless belt onto a bracket superposed over the film and capable of pivoting through 180.degree. in order to overturn the closure strips onto said film. Thus, the strips are initially conveyed with their relief directed towards the film but on the bracket. After which they are turned over so that the relief is directed away from the film and so that they are ready to be fixed thereon. The second solution consists in conveying the strips transversely against a shoe superposed above the film and then in lowering the shoe against the film so as to fix the closure strips in place.
After observing that none of the techniques described in the above-mentioned documents gives satisfaction, proposals were made in document U.S. Pat. No. 5,111,643 for an entirely different approach, consisting not in fixing the closure strips on the film prior to bringing the film to the forming neck as described in the above-mentioned documents, but in initially shaping the film into a tube on the forming neck, and then bringing the closure strips onto the tubular film. For this purpose, document U.S. Pat. No. 5,111,643 proposes a complex installation including a chute which opens out in the base of the forming neck to bring the closure strips which are carried on a support tape, and a chute for taking away the support tape.
Other means designed to place closure strips transversely onto a film are described in document U.S. Pat. No. 4,709,398 and U.S. Pat. No. 4,909,017.
The Applicant has itself defined in its patent application FR-A-2 745 261 a machine for forming film-based packages, which machine has means for bringing closure strips transversely onto a film, which means comprise a combination of a rectilinear guide superposed over the film and extending transversely thereto in order to position at least one closure strip accurately and transversely over the film, together with means for taking hold of the leading end of the closure strip and suitable for moving transversely along the guide so as to bring the closure strip into the guide by pulling on the leading end of the closure strip.